(1) Field of the Invention
The present invention pertains to a fixing device and an image formation device using the fixing device, specifically to a fixing device including a resistance heating layer.
(2) Description of the Related Art
In recent years, greater energy economy than that offered by halogen heaters has been sought for the fixing devices used in image formation devices, such as printers. Propositions include the use of a fixing device having a fixing belt that includes a resistance heating layer (e.g., Japanese Patent Application Publication No. 2009-109997).
FIG. 7 is an overall perspective view diagram of such a fixing device 500.
As shown, the fixing device 500 includes a fixing belt 554, a pressure roller 550, a pressing roller 560, and a pair of power supply rollers 570 connected to an A/C power supply.
The fixing belt 554 is a cylindrical, resilient, and deformable belt that includes a resistance heating layer 554b and has electrodes 554e formed over the resistance heating layer 554b at each end thereof, in the width (Y-axial) direction.
The pressure roller 550 has a metal core 551 covered by a resilient layer 552, and the fixing belt 554 is loosely fit therearound so as to circulate.
A pressing roller 560 is arranged outside the circulation path of the fixing belt 554 and pressurizes the pressure roller 550 through the fixing belt 554, forming a fixing nip 530.
Also, the pressing roller 560 receives a driving force from a (non-diagrammed) driving motor and thus rotates in the direction indicated by arrow P. The driving force is transmitted through the fixing belt 554 to the pressure roller 550, such that the fixing belt 554 and the pressure roller 550 are driven to rotate in the direction indicated by arrow Q.
The pair of power supply rollers 570 is in contact with the electrodes 554e of the fixing belt 554, outside the circulation path, pressing the fixing belt 554 downward, as shown (i.e., in the Z′ direction). Accordingly, power is supplied to the resistance heating layer 554b of the fixing belt 554.
According to this configuration, the electrodes 554e supply power via the power supply roller 570 while the fixing belt 554 is driven to circulate. As this occurs, the electrical resistance in the electrodes 554e is much less than that of the resistance heating layer 554b, to the extent that voltage drops in the electrodes 554e are safely ignored. Thus, electrical current flows over the entire circumference of each electrode 554e and Y-axially across the entire resistance heating layer 554b, producing heat therein.
Given that the direction of the current I periodically reverses, the direction indicated in FIG. 7 for the current I is simply an example at a given point in time.
Here, portions of the fixing belt 554 other than those pressed by the fixing nip 530 and the power supply roller 570 are not in contact with any other components and cannot avoid surrounding heat. As such, temperatures effectively increase in the region of the fixing nip 530 through Joule heating and, when a (non-diagrammed) recording sheet with a toner image formed thereon passes through the fixing nip 530, the heat and pressure fixes the toner image to the recording sheet.
However, when the pressing roller 560 is driven to rotate while being pressed by the pressure roller 550 through the fixing belt 554, the friction between the power supply roller 570 and the fixing belt 554, which is deformed into an oval, leads to unstable running conditions for the fixing belt 554 and may cause running path fluctuations (hereinafter, ripples) in slack portions occurring in the fixing belt 554.
FIGS. 8A and 8B illustrate a somewhat exaggerated aspect of the ripple phenomenon occurring on the fixing belt 554.
As shown in FIG. 8A, the power supply roller 570 is normally pressed toward the pressure roller 550 by a compressed spring 571 or similar. When a rise portion 554a forms in the slack portion, having risen away from the pressure roller 550, regains its original position upon passing through, and the motion of the fixing belt 554 follows with some delay, then as shown in FIG. 8B, a space S may be produced between the power supply roller 570 and the fixing belt 554.
As such, the potential difference between the power supply roller 570 and the fixing belt 554 produces a spark across the space S, which is problematic in that a small hole may be opened thereby in the surface of the fixing belt 554, reducing the useful life thereof.